(a) Field of the Invention
The present invention relates to a system and method for controlling a hybrid vehicle that controls a rotational speed of an engine to enable starting of the engine in a hybrid system that includes an engine, a first motor-generator, and a second motor-generator.
(b) Description of the Related Art
Generally, automatic transmissions use hydraulic pressure to shift gears in a multi step process to output the appropriate torque from a rotational torque of an engine/motor according driving conditions and driver demand. Some hybrid vehicles utilize two motor/generators (MG) and one engine that are connected through a planetary gear set and control the motor/generator to achieve a continuous variable shifting system or CVT.
The engine, the first and second motor/generators, and two planetary gear sets are used to continuously vary the output speed of a transmission according to driving conditions of the vehicle and driver demand. In particular, each speed of the first and second motor/generators are controlled, accordingly.
More specifically, a CVT can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This is much different than the traditional mechanical transmission that has a fixed number of gear ratios. A CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel efficiency than other types of transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. CVT's also can maximize the performance of a vehicle by allowing the engine to turn at an RPM which produces peak power. Finally, a CVT does not strictly require the presence of a clutch, allowing for a clutch to be omitted from the overall system. By omitting the clutch, maintenance costs and manufacturing costs can be significantly reduced.
In these types of systems, the first motor/generator is often speed controlled according to the driving condition of the engine and the second motor/generator is torque controlled together with the engine to control the entire output torque. In a neutral mode, however, an engine is separated from a wheel shaft and all rotational elements (i.e., the wheels) are no longer forcible rotated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.